1. Field of Invention
The present invention relates to piezoelectric resonators which are mounted on the surface of an electronic appliance, especially to a package structure in which a piezoelectric resonator piece is mounted in a cantilever manner and hermetically sealed.
2. Description of Related Art
Piezoelectric resonators are extensively used as a clock source for electronic circuits in a variety of electronic appliances including information communication devices such as cellular phones and PHSs, OA appliances such as computers, and daily necessities such as electronic watches. Attempts are currently being made to produce smaller piezoelectric resonators in size and thickness according to the realized compactness and small thickness of electronic appliances, particularly those which are portable.
In general, surface mounting type piezoelectric resonators may include a piezoelectric resonator piece, for instance quartz crystal, which is supported with one end in a cantilever manner, enclosed in a package made of an insulating material such as ceramic, and hermetically sealed. FIG. 11 shows an embodiment of a surface mounting type quartz crystal resonator in which a tuning fork type quartz crystal resonator 4 is enclosed in a package 3 which consists of a flat, rectangular base 1 and a thin panel lid 2. The tuning fork type quartz crystal resonator 4 having excitation electrodes on its quartz surface is mounted in a cantilever manner in which a base end 4a having leading electrodes from the excitation electrodes is fixed to electrode pads 5 disposed on the inner bottom surface of the base 1 using conductive adhesive 6. Then, the lid 2 is engaged on top of the base 1 and hermetically sealed.
The conventional piezoelectric resonators described above have a problem with resistance to impact, resulting from their compactness and small thickness. For instance, Patent Publication H7-354228 discloses the structure of a piezoelectric resonator mounted in a cantilever manner in which one end of the piezoelectric resonator piece is fixed to one of two platforms which are disposed on the inner bottom of the package so as to face to each other, and the other free end is slightly spaced from the other platform. As a result, the distance between the upper surface of the piezoelectric resonator piece and the inner surface of the lid is reduced to less than a predetermined value. When the piezoelectric resonator piece is subject to an impact, such as a fall, the free end of the piezoelectric resonator piece will touch the upper end of the platform or the inner surface of the lid, suppressing its up and down movement and absorbing the shock, thereby improving its resistance to impact.
Patent Publication H10-23882 describes a structure where the package in which a piezoelectric resonator piece is supported in a cantilever manner has a slanted, inner bottom surface. The free end of the piezoelectric resonator piece is slightly spaced from the higher part of the slanted bottom surface. With this structure, external impact can be absorbed by the free end of the piezoelectric resonator piece touching the inner bottom surface of the package. Consequently, both the resistance to impact and a reduction in the thickness of the package can be realized.
The present inventors found that the free end 4b of the tuning fork type quartz crystal resonator 4 is largely dislocated upward or downward as is shown by the imaginary line 4xe2x80x2 in FIG. 11 when the piezoelectric resonator is subject to a significant external impact such as a fall. Then, the free end 4b touches the inner surfaces of the package 3, that is the inner bottom surface of the base 1, or the inner surface of the lid 2, and loosens the corner material. This is the main cause of changing the mass of the tuning fork arms and consequently the frequency. The free end 4b of the piezoelectric resonator piece may be inclined upward or downward when its base end 4a is fixed onto the electrode pad 5. It is highly possible that the free end 4b hits the bottom surface of the base 1 upon the impact of a fall and breaks, particularly if it is inclined downward. The present invention is proposed at least based on the findings described above.
On the other hand, debris falling off of the arm tips of the tuning fork may become deposited on the tuning fork type quartz crystal resonator piece 4. This may cause frequency change and further deterioration in resonator properties. Therefore, the prior art obtains resistance to impact by mounting the piezoelectric resonator piece at a level distant from the inner bottom surface of the base and positioning the lid at a higher level, so that a large space is produced enough for the free end of the piezoelectric resonator to be dislocated upward or downward on impact without touching the inner surfaces of the package. However, this configuration results in increasing the total thickness of the piezoelectric resonator and does not satisfy the compactness and flatness requirements.
For piezoelectric resonators of the tuning fork type, frequencies are finally adjusted by partially fusing and removing the metal weight formed on the arm tips of the tuning fork using a laser beam emitted from an external source through a transparent glass lid after the piezoelectric resonator piece is sealed in a package. Compactness and flatness are realized by reducing the space between the piezoelectric resonator piece and the lid. Metal fusion by the laser beam is easily re-deposited on the tuning fork arms and affects the frequency, hampering the fine frequency adjustment.
An object of the present invention is, in response to the compactness and flatness requirements for surface mounting type piezoelectric resonators, to provide a package structure which at least has excellent resistance to impact without increasing the package thickness.
Another object of the present invention is to provide a package structure which allows at least fine frequency adjustment, particularly in piezoelectric resonators of the tuning fork type.
The package structure for a piezoelectric resonator according to various exemplary embodiments of the present invention consists of at least achieving the above described purposes. In accordance with various exemplary embodiments, a piezoelectric resonator piece is provided which is mounted in a cantilever manner and hermetically sealed in package structure in which the package consists of a base having a mounting surface provided with connection electrodes on which the piezoelectric resonator piece is mounted with one end in a cantilever manner using conductive adhesive, and a lid which is engaged with the upper part of the base. In these various exemplary embodiments, either one or both of the mounting surface of the base and the inner surface of the lid have a recess formed adjacent to the free end of the piezoelectric resonator piece.
With the above structure, the free end of the piezoelectric resonator piece does not touch the mounting surface of the base and/or the inner surface of the lid when it is largely dislocated upward or downward upon impact, from such as a fall. The factors which may cause frequencies to deviate from a predetermined range can be eliminated, such as tip fracture and tip debris depositing on the piezoelectric resonator piece. In addition, the space between the piezoelectric resonator piece and both the mounting surface of the base and the inner surface of the lid can be reduced. There is no need for the entire package to have a large thickness.
Particularly, piezoelectric resonators of tuning fork type can be advantageous because frequency fluctuation due to the mass change of the tuning fork arms is prevented. Furthermore, the accuracy of frequency is improved as a result of the fused metal splashes being deposited on the inside of the recess, not on the tuning arms when the metal weight is fused and removed to adjust the frequency by directing a laser beam onto the arm tips through a lid, made of, for instance, transparent glass, after the package is sealed. Impairment of the inner bottom surface of the recess by laser beams which have passed the tuning fork arms is prevented, because it is more distant from the focal point of the laser beam compared to the conventional way.
In an exemplary embodiment of the present invention, the recess formed on the mounting surface of the base has a slanted wall on the side closest to the connection electrodes. With this, the impact on the middle part of the piezoelectric resonator piece touching the recess edge is alleviated when it is largely dislocated upward or downward, and also an increased amount of dislocation can be accepted.
The recess can have a variety of shapes in its top view, such as rectangular, oval, or track (like an athletic sports track) in accordance with the free end of the piezoelectric resonator piece. The rectangular shape will allow a large surface area for the recess. The comers of the rectangular are preferably rounded so that the recess is easily processed and the package strength is improved as a result of alleviating the stress concentration on the comers, and preventing cracks in the course of forming a recess and assembling a piezoelectric resonator, as well as in its use. The oval shape improves the package strength by preventing the entire stress on the package from concentrating around the recess. The track shape will provide these two advantages by preventing the entire stress on the package from concentrating around the recess and allowing a large surface area.
It is preferable that a communication channel be provided in the recess formed on the mounting surface of the base during the last stage of manufacturing in order to vacuum the inside of the package, because this will make the package structure simple and reduce the number of production steps.
In order to greatly reduce the chance of breakage followed by frequency change of the piezoelectric resonator piece upon the impact of a fall, and also to realize the compactness and flatness of the package, it is convenient that the package structure be sized to satisfy the expressions: 0.5 less than L1/L0xe2x89xa61, wherein L0 is the distance between the fixed supporting point and the free end tip of the piezoelectric resonator piece mounted on the connection electrodes, and L1 is the distance between the fixed supporting point and the edge of the recess which is on the side closest to the connection electrodes; and t2/t1xe2x89xa70.8, wherein t1 is the height to the lid from the upper surface of the piezoelectric resonator piece, and t2 is the height to the piezoelectric resonator piece from the bottom surface of the recess.